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NCAR / CfRadial

NetCDF CF Conventions for radial coordinate data for RADAR and LIDAR
BSD 2-Clause "Simplified" License
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Sweep time coordinate relative to time_coverage_start may be problematic #10

Open marack opened 5 years ago

marack commented 5 years ago

The time coordinate variable for a sweep is currently described as "Time at center of each ray, in fractional seconds since time_coverage_start". I think this specification may cause problems for systems that need to send data on a sweep-by-sweep basis.

A very common use case (for me) is to send a volume as a series of single sweep files. This allows early viewing of the data while the volume is still being scanned. After all sweeps are received, the individual files are combined into a single volume.

By specifying the ray times as relative to time_coverage_start, and time_coverage_start as the time of the first ray in the file this means that the times in each sweep file will be relative to a different point. When combining sweeps into a single volume file it would be necessary to rewrite all of the time coordinates to be relative to the time_coverage_start of the earliest sweep.

I would like to propose that we remove the "in fractional seconds since time_coverage_start" part of the description for this variable in section 5.2 (page 24). The variable already has the units well specified as 'seconds since YYYY-MM-DD hh:mm:ss'. The user should be free to use any appropriate timestamp in this unit string rather than being forced to use the time corresponding to time_coverage_start.

In the sweep-by-sweep scenario above this would allow files to be combined without forcing a translation of all time coordinates to be relative to the earliest sweep.

mike-dixon commented 5 years ago

That sounds OK - I can go with that. When you send individual sweep files, I assume you change time_coverage_start and _end to refer to the sweep instead of the volume, is that correct?